Ising model on a square lattice

We plot a local magnetization (up or down) as a color
block in a square lattice. Here, the system size is \(L=96\). We
update the spin configuration with one click for 100 MCS for
Metropolis and Swendsen-Wang algorithms and 100 steps for Wolff
algorithm. The vertical position of a clicked pointer controls the
value of parameter \(z (\equiv \exp(2J/T) - 1)\). The horizontal
position also selects a Monte Carlo algorithm. The left, center, and
right cases are Metropolis, Swendsen-Wang, and Wolff algorithms,
respectively.

In the Ising model, we fix an atom at a lattice site and the local
magnetization of atom takes only two opposite directions like north or
south. An atom interacts only with nearest neighbor atoms so that the
local magnetization and that of interacting atoms prefer the same
direction when the temperature falls. Interestingly, a local
magnetization of each atom simultaneously start to take the same
direction at a certain temperature. Thus, the total magnetization
continuously appears at the certain temperature. This is a well-known
example of continuous phase transition.

In present demonstrations of Markov Chain Monte Carlo
(MCMC) simulations, we can see states of the Ising model on the
two-dimensional square lattice at a temperature \(T\). At a low
temperature (\(z \ge \sqrt{2}\)), a big island appears. It
corresponds to the spontaneous total magnetization. We can also change
algorithms (Metropolis, Swendsen-Wang,
and Wolff ) which generates the states. In particular,
Swendsen-Wang or Wolff algorithms can sample various states by the
small number of steps. These algorithms are often used in recent MCMC
simulations of Ising model.